High frequency amplifiers operate most efficiently when the output of the amplifier is delivered to an impedance matched load, that is, a load that has an input impedance equal to the output impedance of the amplifier. However, the output impedance of a high frequency amplifier is a function of both the frequency of operation of the amplifier as well as the power output by the amplifier. Thus, to obtain high efficiency at a given operating frequency, it is desirable to adapt the impedance of the load based on the power output by the amplifier. For example, some digital modulation techniques have a high peak-to-average power ratio. A designer of an RF power module may choose to optimize the amplifier chain for efficiency at the peak output power. However, in that case, the efficiency of the amplifier at the average power level may suffer.
Varactor diodes, also referred to as varicap diodes, variable capacitance diodes, and tuning diodes, can be used as variable reactance elements in an impedance matching transformer for a high frequency amplifier. The capacitance of a varactor diode can be controlled by adjusting the reverse bias level of the diode. Because varactor diodes are operated in reverse bias, only limited current flows through the diode during such operation. However, since the thickness of the depletion region in a reverse biased diode varies with the applied bias voltage, the capacitance of the diode can be controlled. In a conventional diode structure, the depletion region thickness is proportional to the square root of the applied voltage, and the capacitance of the diode is inversely proportional to the depletion region thickness. Thus, the capacitance of a conventional diode is inversely proportional to the square root of applied voltage.
The performance of a varactor is also characterized by its so called Q factor, defined as the ratio between the capacitive reactance to the equivalent series resistance (ESR). To achieve high Q (i.e., low loss performance), the material used to form the varactor should be able to support a high control voltage while maintaining low resistance. The wide bandgap materials including, but not limited to SiC or GaN, have an inherently high electric field strength, allowing a thin highly doped layer to have a large breakdown voltage. Accordingly, such materials should allow a very high Q value to be achieved with low insertion loss.
Some attempts have been made to alter the capacitance-voltage relationship of silicon or gallium arsenide-based varactor diodes by providing a graded doping profile in the voltage blocking region of the diode. The graded doping profile has been typically achieved either using epitaxial growth, diffusion or implantation. However, providing a complex graded doping profile can prove difficult in practice, because of the limitations of the processes used in reproducible manufacturing.